Ornithology of Northern Africa. 431 
complexion than the others. These, and such of their offspring 
as most resembled them, would become more liable to capture 
by their natural enemies, hawks and carnivorous beasts. The 
lighter-coloured ones would enjoy more or less immunity from 
such attacks. Let this state of things continue for a few hun¬ 
dred years, and the dark-coloured individuals would be exter¬ 
minated, the light-coloured remain and inhabit the land. This 
process, aided by the above-mentioned tendency of the climate 
to blanch the coloration still more, would in a few centuries pro¬ 
duce the Galerida abyssinica as the typical form. And it must 
be noted, that between it and the European G. cristata there is 
no distinction but that of colour. 
But when we turn to Galerida isabellina , G. arenicola, and 
G. macrorhyncha , we have differences not only of colour but of 
structure. These differences are most marked in the form of 
the bill. Now to take the two former first. G. arenicola has 
a very long bill, G. isabellina a very short one ; the former resorts 
exclusively to the deep, loose sandy tracts, the latter haunts the 
hard and rocky districts. It is manifest that a bird whose food 
has to be sought for in deep sand derives a great advantage from 
any elongation, however slight, of its bill. The other, who 
feeds among stones and rocks, requires strength rather than 
length. We know that even in the type-species, the size of the 
bill varies in individuals, in the Lark as well as in the Snipe. 
Now, in the Desert, the shorter-billed varieties would undergo 
comparative difficulty in finding food where it was not abundant, 
and consequently would not be in such vigorous condition as 
their longer-billed relatives. In the breeding-season therefore 
they would have fewer eggs and a weaker progeny. Often, as 
we know, a weakly bird will abstain from matrimony altogether. 
The natural result of these causes would be that in course of time 
the longer-billed variety would steadily predominate over the 
shorter, and in a few centuries they would be the sole existing 
race, their shorter-billed fellows dying out until that race was 
extinct. The converse will hold good of the stout-billed and 
weaker-billed varieties in a rocky district. 
Here are only two causes enumerated which might serve to 
create as it were a new species from an old one, yet they are 
perfectly natural causes, and such as, I think, must have occurred, 
